Mycosis fungoides and the Sézary syndrome are diseases in which lymphocytes (a type of white blood cell) become malignant (cancerous) and affect the skin.

Normally, the bone marrow makes blood stem cells (immature cells ) that develop into mature blood stem cells over time. A blood stem cell may become a myeloid stem cell or a lymphoid stem cell. The myeloid stem cell develops into a red blood cell, white blood cell, or platelet. The lymphoid stem cell develops into a lymphoblast and then into one of three types of lymphocytes (white blood cells):

• B-cell lymphocytes that make antibodies to help fight infection.
• T-cell lymphocytes that help B-lymphocytes make the antibodies that help fight infection.
• Natural killer cells that attack cancer cells and viruses.

Blood cell development. A blood stem cell goes through several steps to become a red blood cell, platelet, or white blood cell.

In mycosis fungoides, T-cell lymphocytes become cancerous and affect the skin. In the Sézary syndrome, cancerous T-cell lymphocytes affect the skin and the peripheral blood.

Mycosis fungoides and the Sézary syndrome are types of cutaneous T-cell lymphoma.

This summary describes the two most common types of cutaneous T-cell lymphomas: mycosis fungoides and the Sézary syndrome. For information about other types of skin cancer or non-Hodgkin lymphoma, see the following PDQ summaries:

• Adult Non-Hodgkin Lymphoma Treatment
• Skin Cancer Treatment
• Melanoma Treatment
• Kaposi Sarcoma Treatment

A possible sign of mycosis fungoides and the Sézary syndrome is a red rash on the skin.

Mycosis fungoides and the Sézary syndrome may move through the following phases:

• Premycotic phase: A scaly, red rash in areas of the body that usually are not exposed to the sun. This rash does not cause symptoms and may last for months or years. It is hard to diagnose the rash as mycosis fungoides during this phase.
• Patch phase: Thin, reddened, eczema-like rash.
• Plaque phase: Thickened, red patches or reddened skin.
• Tumor phase: Tumors form on the skin. These tumors may develop ulcers and the skin may get infected.

Sézary syndrome is an advanced form of mycosis fungoides.

In the Sézary syndrome, skin all over the body is reddened, itchy, peeling, and painful. There may also be patches, plaques, or tumors on the skin. Cancerous T-cells are found in the blood. Mycosis fungoides does not always progress to the Sézary syndrome.

Tests that examine the skin and blood are used to detect (find) and diagnose mycosis fungoides and the Sézary syndrome.

The following tests and procedures may be used:

• Physical exam and history: An exam of the body to check general signs of health, including checking for signs of disease, such as lumps, the number and type of skin lesions, or anything else that seems unusual. Pictures of the skin and a history of the patient’s health habits and past illnesses and treatments will also be taken.
• Complete blood count with differential: A procedure in which a sample of blood is drawn and checked for the following:
  • The number of red blood cells and platelets.
  • The number and type of white blood cells.
  • The amount of hemoglobin (the protein that carries oxygen) in the red blood cells.
  • The portion of the blood sample made up of red blood cells.
  Complete blood count (CBC). Blood is collected by inserting a needle into a vein and allowing the blood to flow into a tube. The blood sample is sent to the laboratory and the red blood cells, white blood cells, and platelets are counted. The CBC is used to test for, diagnose, and monitor many different conditions.
• Peripheral blood smear: A procedure in which a sample of blood is viewed under a microscope to count different circulating blood cells (red blood cells, white blood cells, platelets, etc.) and see whether the cells look normal.
• Biopsy: The removal of cells or tissues so they can be viewed under a microscope to check for signs of cancer. The doctor may remove a growth from the skin, which will be examined by a pathologist. More than one skin biopsy may be needed to diagnose mycosis fungoides.
• Immunophenotyping: A process used to identify cells, based on the types of antigens or markers on the surface of the cell. This process may include special staining of the blood cells. It is used to diagnose specific types of leukemia and lymphoma by comparing the cancer cells to normal cells of the immune system.
• Immunogenotyping: A procedure in which a sample of DNA from a skin biopsy is studied to see if the genes for certain kinds of immune system proteins, such as the T-cell receptor or antibody proteins, are arranged in one pattern. Normally T-cell receptor genes and antibody genes are arranged in many different patterns. In mycosis fungoides and the Sézary syndrome, the genes are arranged in a single pattern.

Certain factors affect prognosis (chance of recovery) and treatment options.

The prognosis (chance of recovery) and treatment options depend on the following:

• The stage of the cancer (the amount of skin affected and whether cancer has spread to the lymph nodes, the blood, or other places in the body).
• The type of lesion (patches, plaques, or tumors).
• The number of cutaneous T-cell lymphocytes in the blood.

Mycosis fungoides and the Sézary syndrome are difficult to cure. Treatment is usually palliative, to relieve symptoms and improve the quality of life. Patients can live many years with this disease.